In recent years, as a technology for improving fuel efficiency and the environmental protection performance of a vehicle, introduction of a bi-fuel engine system, which selectively switches between a liquid fuel, such as gasoline, and a gaseous fuel, such as compressed natural gas (CNG), and supplies the fuels to a single engine, has progressed. Generally, in this bi-fuel engine system, in a case of using the gaseous fuel, the highly pressurized gaseous fuel that is filled in a gas tank is decompressed to a predetermined pressure by a regulator and is supplied to a fuel injection valve that is dedicated to the gaseous fuel.
An electromagnetic type shut-off valve is inserted in a fuel supply path ranging from the gas tank to the regulator, and initiation and stop of the gaseous fuel supply may be switched by controlling the open and shut states of the shut-off valve using a control device. A shut-off valve fault may have a significant adverse effect on the entirety of a system, such that various technologies for diagnosing shut-off valve faults have been developed in the related art. For example, PTL 1 discloses a technology in which a valve-shutting control of a shut-off valve is performed during operation of an engine, and in a case where the amount of a drop in a gaseous fuel pressure after the control exceeds a reference value within a predetermined diagnosis time, it is determined that the shut-off valve is in a shut valve state (normal). On the other hand, in a case where the amount of the drop does not exceed the reference value, it is determined that the shut-off valve is fixed-opened (abnormal).